1. Field of the Invention
This device relates to an insulated wire for a winding suited to be used in a high frequency transformer, a high frequency reactor, or a high frequency coil used in such devices such as a switching power source.
2. Description of the Prior Art
Generally a magnet wire manufactured by forming an insulating layer made of such a material as polyurethane resin or polyester resin on a single wire conductor has been used as an insulated wire for a winding for a switching power source.
A safety transformer for such a device as a switching power source must follow the following restrictions based on IEC (International Electrotechnical Commission) or UL (Standards of Underwriter's Laboratories, Inc.,) as well as on other various types of safety standard;
(1) An insulation resistance must be provided between layers of an electric wire or between the primary and secondary windings with the help of a specified insulating film.
(2) To secure a creepage distance between a winding and a core, a space insulation must be provided with an insulating barrier between the winding and the core.
(3) It is necessary to carry out a processing for insulation by using such a material as an insulating tube when connecting a lead wire to a pin terminal.
Because of the restrictions required by the safety standards as described above, when using a magnet wire, sometimes the user may face many troubles such as difficulty in minimizing a transformer, requirement for parts and processes to carry out various types of processing for insulation, or difficulty in obtaining a compact and high performance transformer. To solve these problems, it has been proposed to use a 3-layered insulated wire for a winding to be used in a transformer, as described in Japanese Utility Model Application No. 49802/1990, Japanese Patent Application No. 150174/1990, and Japanese Utility Model Application No. 49801/1990, and now it is possible to satisfy the safety standards such as IEC or UL.
In a switching power source, a high frequency in a range from several tens KHz to several hundreds KHz is now used for a switching frequency to improve the switching efficiency.
However, in such a high frequency band area as described above, an eddy current loss in a conductor of a transformer winding and a loss due to the skin effect become very large, which causes heat emission from a transformer and may degrade characteristics of not only an insulated electric wire for a winding, but also a transformer itself.